1. Field of the Invention
The present invention relates to a scanning probe microscope according to the four-tip probe method which the microscope is to be used in, for example, semiconductor process evaluation, and also to a probe suitable for analyzing an ultra-surface region of a sample when used in the scanning probe microscope.
2. Description of Related Art
The invention of the transistor evolved from studies of the electrical characteristics of a semiconductor surface, particularly the surface electron state. However, with respect to electrical conduction due to the state of surface electrons themselves, many points have been left unanalyzed until today. This “surface state conduction” is extremely difficult to measure because electricity runs only in one or two electron layers of a crystal surface. However, thanks to the development of new measuring and inspecting techniques, such as a four-tip probe scanning tunnel microscope operating in an ultra-high vacuum and a microscopic four-tip probe, direct measurement of the surface state conduction has become possible and very interesting conduction characteristics have become revealed as a result. To this end, it has been determined that the electron state of a semiconductor surface has a unique characteristic totally different from that of the bulk state. In the electron device field, apparatuses of this type will play an important role in research and development of electron devices.
In an evaluating apparatus using a scanning tunnel microscope according to the four-tip probe method, four probe tips are arranged linearly at regular distances, a current is caused to flow into a sample from the outer two of the probe tips, and a voltage drop caused due to the electrical resistance of the sample is measured by the inner two of the probe tips. At such time, because there is only a very slight current flowing in these probe tips, only a voltage drop V on the sample can be measured without influence of the contact resistance at a point of contact of the probe tips with the sample. An electrical resistance according to the four-tip probe method is obtained by R=V/I where I is a measured current. As shown in FIGS. 10A and 10B there is a correlation between the inter-probe-tip distance and the depth of probing into a sample. In order to obtain information of the ultra-surface of the sample, it is essential to arrange the probes at the corresponding narrow distances shown in FIG. 10B. In the related art, however, there is a limit in processing. This is to say that the diameter of the individual probe tip has served as a restriction so that a probe having a probe-tip pitch of several μm could not be manufactured.
Conventionally, four-tip probes whose inter-tip distance is in the order of millimeters to centimeters have been used, and many studies on this type of probe were carried out. However, these conventional probes cannot be applied to surface analysis of semiconductor devices. Recently, an undergraduate research group of Tokyo University released a report (Applied Physics, 70th Volume, 10th Issue, 2001) on measurement of electrical resistance of a silicon crystal surfaces using a microscopic four-tip probe of a several μm pitch manufactured utilizing silicon micro-processing technology, such as ordinary lithography. For analyzing the outermost or uppermost device-surface, however, this several μm inter-tip distance is inadequate to achieve proper performance. An inter-tip distance of at most 1 μm or less is needed for doing so. Even if the above-mentioned silicon micro-processing technology is employed, it is difficult to manufacture a four-tip probe having an inter-tip distance of a such a sub-micron order.
In a further related art study, positioning of measuring points on an object surface is carried out using an optical microscope. However, because a required measuring region for analyzing the outermost or uppermost device-surface is extremely small, it is difficult to achieve positioning using the conventional optical microscope and, as an alternative means, a new observation technique, such as a scanning electron microscope (SEM) and an atomic force microscope (AFM) has been required. When an SEM is used, a sample is always irradiated with electrons during observation. This may produce noise and render accurate measurement of electricity impossible. On the other hand, in the case of an AFM, observation can be realized either in an ordinary atmospheric environment or a special atmospheric environment. However, when a multi-tip probe itself is used also as an image-obtaining probe, this may be a hindrance to accurate measurement for reasons such as (1) it is difficult to perform image analysis from signals detected by a plurality of probe tips arranged in a row and (2) the image is contaminated or otherwise damaged by scanning. Further, in the conventional AFM, it is a common practice to employ the light leverage method in which a mirror is mounted on a cantilever to detect displacement. In this case, a sample is irradiated with laser light. Because laser light serves as an excitation energy source to cause surface atoms to enter an excited state, this has a considerable effect on the movement of electrons on a device surface and therefore also impedes accurate measurement of electricity. Alternatively, waves serving as excitation light can be removed by wavelength cutoff using a filter. However, this alternative cannot realize observation in a perfect dark field and would often encounter problems, such as decreases in sensitivity due to attenuation of light intensity.